The present invention relates to friction clutches in general, especially to improvements in friction clutches which can be used with advantage in motor vehicles. More particularly, the invention relates to improvements in friction clutches of the type wherein a pressure plate is biased against a clutch disc by a diaphragm spring which is installed between two seats and wherein the diaphragm spring is secured to the cover in such a way that it can be tilted between the two seats while the clutch changes its state from engaged to disengaged condition or vice versa. Still more particularly, the invention relates to improvements in friction clutches of the type wherein the means for coupling the diaphragm spring to the cover of the friction clutch extend axially through and inwardly beyond the diaphragm spring.
It is well known to mount the diaphragm spring on the cover of a friction clutch in such a way that the diaphragm spring is tiltable between two seats. For example, it was already proposed to secure the diaphragm spring to the cover by rigid fasteners in the form of rivets or in the form of integral portions of the cover. A drawback of such proposal is that the diaphragm spring is invariably installed with some play if the cover, the diaphragm spring, the seats and/or the fasteners are machined with a certain amount of tolerance, i.e., if the dimensions of such parts deviate from optimum dimensions. Alternatively, the diaphragm spring is held between the two seats with an excessive force so that it cannot be readily tilted for the purpose of engaging or disengaging the friction clutch. The just outlined situation can arise if the rivets which secure the diaphragm spring and the seats to the cover are too short. In either event (i.e., if the diaphragm spring is held between the seats with a certain amount of play or if the amount of play is zero and the diaphragm spring is clamped between the seats with an excessive force), the efficiency of the friction clutch is less than satisfactory especially if the diaphragm spring is installed between the two seats with a certain amount of play so that the clutch release means must cover a greater distance than necessary in order to change the state of the clutch from engaged condition to disengaged condition or vice versa. If the diaphragm spring is clamped between the seats with an excessive force, the wear upon the spring and upon the seats rapidly reaches a value at which the friction clutch must be repaired or discarded. Excessive clamping of the diaphragm spring between the seats also leads to excessive frictional hysteresis which, in turn, entails excessive flexing of the prongs which form part of the diaphragm spring and must be displaced by a bearing or the like in order to engage or disengage the clutch.
Another drawback of presently known systems for securing the diaphragm spring between two seats at the inner side of the cover in a friction clutch is that the play between the diaphragm spring and the seats invariably increases as a result of wear upon such parts so that the distance through which the prongs of the diaphragm spring must be flexed or shifted in order to engage or disengage the clutch must be increased gradually which is undesirable for obvious reasons, especially when the friction clutch is installed in a motor vehicle to transmit torque between the crankshaft of the internal combustion engine and the wheels. The wear entails a rapid increase of play between the diaphragm spring and its seats, even if the play is zero when the friction clutch is new.
Heretofore known friction clutches wherein the means for holding the diaphragm spring at the inner side of the cover constitute integral parts of the cover also exhibit a number of serious drawbacks. This applies irrespective of whether or not the integral portions of the cover bear directly against the inner side of the diaphragm spring. For example, it was already proposed to construct rivets which connect the diaphragm spring with the cover in such a way that the rivets exhibit shoulders for the diaphragm spring. The main drawback of such types of coupling means for the diaphragm spring is that they, too, enable the diaphragm spring to move between its seats with a progressively increasing play as the wear upon the diaphragm spring, its seats, the cover and the coupling means progresses so that the distance through which the clutch engaging or disengaging means must be moved in order to change the condition of the clutch must be increased from time to time during the useful life of the friction clutch.
Attempts to overcome the just discussed drawbacks of conventional friction clutches include the provision of one or two resilient seats which are intended to compensate for wear upon the diaphragm spring, the seats and/or the coupling means and to thus ensure that the distance which the clutch actuating device must cover will remain the same irrespective of the age and extent of wear upon the clutch. The bias of the resilient seat or seats must exceed the force which is applied in order to change the condition of the friction clutch, i.e., such bias must exceed the force which the diaphragm spring exerts upon the resilient seat or seats during a change in the condition of the friction clutch. This contributes significantly to the useful life of the clutch and to more predictable operation during engagement or disengagement because the wear upon the seats, upon the diaphragm spring and upon the means for coupling the diaphragm spring to the cover is compensated for by the resiliency of the seat or seats. The just outlined advantages can be achieved by employing at least one resilient seat or by employing an additional spring which is installed between the coupling means and one of the seats, between one of the seats and the diaphragm spring or between one of the seats and the cover. Such additional spring must be adequately stressed during assembly of the diaphragm spring with the seats and the cover of the friction clutch. The additional spring or each of the resilient seats can resemble or constitute a second diaphragm spring which preferably constitutes that seat that faces the inner side of the main diaphragm spring, i.e., that side which faces away from the inner side of the cover. Such seat in the form of a second diaphragm spring can be secured to the cover by rivets, by other types of fasteners or by integral portions of the cover. A drawback of this proposal is that the second diaphragm spring contributes significantly to the overall cost of the friction clutch, not only because it must be constructed, configurated and dimensioned with a high degree of precision but also because it must be mounted in the friction clutch with a view to ensure that it undergoes the right amount of initial stressing.